The invention relates to a method and to a device for driving optical laser diode(s) in an optical communication network.
In addition, an according optical amplifier and a communication system are suggested.
Optical amplifiers are used in optical communication networks to intensify, e.g., optical signals that are attenuated along the fiber-optic communication path within optical communication networks.
In fiber-optic communication networks, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.
Raman amplification as one example of optical amplification is based on the Stimulated Raman Scattering (SRS) phenomenon, when a lower frequency signal induces an inelastic scattering of a higher-frequency pump photon in an optical medium in the nonlinear regime. As a result of this, another lower-frequency signal photon is produced and the higher-frequency pump photon is annihilated, with the surplus energy resonantly passed to the vibrational states of the medium. This process, as with other stimulated emission processes, allows all-optical amplification.
Raman amplification is an important technology to increase a maximum distance covered by long-haul optical transmission systems and is a suitable technology to supplement erbium-doped fiber amplifiers (EDFAs) commonly used in commercial installations. According to its large bandwidth, Raman amplification is compatible with the requirements of wavelength-division multiplexing (WDM) networks. One of the main advantages of Raman amplification is the usage of transmission fiber as gain medium, providing distributed amplification.
The trend to higher data rates per wavelength channel requires an improvement of the optical signal-to-noise ratio (OSNR) performance of optical transmission links or spans. In many scenarios, an insertion of additional inline amplifiers is not possible. Hence, Raman amplification is expected to be applied more frequently in future optical networks.
A Raman amplifier is an optical amplifier based on Raman gain, which results from the effect of stimulated Raman scattering. An input signal can be amplified while co-propagating or counter-propagating with a pump beam. A Raman amplifier is preferably, but not necessarily, pumped with continuous-wave light from a diode laser. Signal amplification can be achieved by transferring power from one or more optical pumps to, e.g., a WDM signal. Direct power transfer is possible, but power transfer can also be provided via some additional lightwaves (also called higher-order pumping).
Pumping lightwaves are usually coupled into the transmission fiber at the end of the link, propagating in opposite direction to the signals. This “counter directional pumping” results in an improved optical signal-to-noise ratio (OSNR).
“Co directional pumping” results in a reduction of the nonlinear fiber effects, achieving the same output power level at the output of the transmission fiber with reduced nonlinear interactions as without Raman amplification.
Transients are power variations of signals, induced by adding or dropping of optical channels, which can be caused, e.g., by fiber breaks or component failure and result in significant performance degradation. Such kind of impairments can be mitigated by launching additional lightwaves into the fiber (“filling laser”) and by keeping the total power coupled into a span almost constant, even if some of the channels are dropped.
FIG. 1 shows an exemplary setup of a Raman amplifier, wherein several wavelengths λ1 . . . λ4 of emitted light (“pump wavelength”) are used to achieve similar gain for all wavelength signals (“gain flatness”). Each pump wavelength λ1 . . . λ4 is provided by a laser pump LP1 . . . 4 comprising two laser diodes LD1,2, wherein the emitted light at the output of the respective laser diodes LD1,2 is combined by a polarization beam combiner PBC within each of the laser pumps LP1 . . . 4. The provided pump wavelengths λ1 . . . λ4 are combined by a multiplexer MUX, injecting a combined signal cs(λ1 . . . λ4) into a transmission fiber TF in opposite direction to a transport signal ts.
Laser diodes, which are used in optical communication networks or optical communication systems typically come in a package that includes a pigtail pre-aligned with the laser and a back facet monitor photo diode. For a simple data transmission, the laser diode used to generate a data signal is driven such that laser light intensity is modulated so that each digital level has a distinct optical power. The optical digital level must be kept constant over time and temperature to ensure network integrity.
FIG. 2 shows a typical laser diode transfer curve 100, also called “laser transfer function”. There are two regions 101, 102 of operation that characterize the laser transfer function 100. A first region 101 is a sub-threshold region, also called “LED region”. In this region spontaneous emission of light occurs. A second region 102, above laser threshold is a laser active region, above which stimulated emission of light occurs. In a linear region 110, the intensity of the light (“output power”) increases linearly with the injection current through the device. This region 110 of the laser transfer function 110 may also be used for digital transmission of information. A key characteristic of the laser transfer function (also referred to as “L.I. Curve” (Output Light vs. Input Current)) are a threshold current 120 and a slope 130 in the linear region 110 beyond the threshold current 120, referred to as a slope efficiency.
The laser basically is a current-to-light converter and the slope efficiency is a conversion factor. An efficiency of the laser is given by a ratio of optical intensity over injection current or power of emitted light over injection current, and the greater the slope efficiency the more efficient, thus providing higher optical power for a given current through the laser diode.
Unfortunately, the laser transfer function changes over time and temperature. With increasing temperature, the threshold current increases and the slope efficiency falls off exponentially. In addition, aging effects cause the threshold current to increase and the slope efficiency to fall off over the life time of the laser.
A laser diode driver translates logic signals from the electrical domain to the optical domain, delivering current to the laser diode optionally monitoring its output, e.g., with a back facet monitor photo diode.
Optical transponders commercially used are sensitive towards changes of the polarization of the emitted light (“wavelengths”) and induced power fluctuation caused by polarization dependent gain (PDG) or polarization dependent loss (PDL). Due to dependencies of the power transfer from the pumps to the transport signal on the respective wavelength-polarization, Raman amplifiers can contribute significantly to PDG or PDL in case of insufficient design or control.
Significant power transfer is mainly possible for copolarized light (lightwaves) whereas the signal gain is almost zero for orthogonally polarized lightwaves.
Due to the polarization dependency, implementation of a single pump lightwave, emitted by a single laser diode is not suitable for signal amplification in WDM systems or networks. Rather a depolarization of the pump is required to achieve polarization independent amplification and performance.
FIGS. 3A and 3B show two schematic diagrams visualizing two different approaches to achieve depolarization of an optical signal provided by a pump, wherein exemplary depolarization of a single wavelength is illustrated. As a further exemplary embodiment (not shown), both setups shown in FIGS. 3A and 3B can be combined into one amplifier. Furthermore, output ports providing signals with different wavelength can be connected to a wavelength combiner (which can be, e.g., an optical multiplexer) as shown in FIG. 1.
The setup according to FIG. 3B is based on a depolarizer 301 implemented, e.g., by a birefrigent optical fiber, which is connected to a laser diode 302. Quite small degrees of polarization (“DoP”) can be achieved by an optimized adaptation of the length of the fiber to the line width of the pump lightwave. As this setup is based on a single laser pump/diode 302, respective implementation is only useful if the pump power provided by the single laser diode 302 is sufficient.
However, due to improvements, the power emitted by pump laser diodes in the wavelength range from 1420 nm to 1480 nm has been increased which allows several implementations of this setup in commercial systems.
Still, the power provided by a single laser diode is insufficient for many applications. Hence, a setup according to FIG. 3A is used.
According to this setup, shown in FIG. 3A, lightwaves provided by two laser diodes 310, 311 with identical or almost identical wavelength are combined by a polarization beam combiner 312 coupling the combined signal with orthogonal polarizations to an output fiber 313. Thereby the amount of total pump power concerning a single wavelength at the output of the polarization beam combiner 312 is almost two times the power of a single laser diode.
Since differences in power levels will cause PDG or PDL, the main challenge of this setup is the adjustment of the optical power of the light, emitted by both laser diodes 310, 311 in such a way that the power of emitted light is substantially equal for both orthogonal polarizations.
The same applies to setups to mitigate transient effects.
FIG. 4 shows in a block diagram an example for a common approach to eliminate the negative effects of PDG and PDL based on a setup shown in FIG. 3A by implementing monitoring devices 401, 402 in both input branches of the PBC 403 and calibrating the monitors devices 401, 402 at the end of the manufacturing process accordingly. They are calibrated in such a way, that they measure the power of the respective laser diode that is provided at the output of the PBC although they are connected to the input power. With that, the respective level of optical power of each of the optical signals (polarizations) emitted by the respective laser diode 404, 405 can be determined, wherein a driving current 406, 407 for the respective laser diode 404, 405 can be controlled and adjusted accordingly.
Known solutions as mentioned above do have some significant drawbacks:                Two optical monitors are required, one per laser diode, each monitor comprises respectively a photo diode, an optical coupler and an optical splice which involves additional costs;        As the monitors are usually calibrated during the manufacturing process, variations in the transfer function caused, e.g., by component aging or temperature deviations have not been considered. As a consequence, according to an exemplary worst case scenario, the optical power of the light emitted via one of the input branches of the PBC would decrease (“insertion loss”) which causes non-identical power levels of the polarizations at the output of the PBC;        Polarization maintaining couplers and splices are required, which causes significant technical complexity in the manufacturing process.        